The maturation of mathematical reasoning skills is a hallmark of human cognitive and academic development. Cognitive, developmental, educational and clinical psychologists have provided valuable insights into the complex and dynamic developmental changes in mathematical skills during childhood. The ages between 7 and 10 (grades 2, 3 and 4) represent an important period for the development of mathematical skills. However, very little is currently known about the development of brain systems that mediate mathematical skill acquisition. The overarching goal of our proposal is to investigate the development of mathematical skills in children using a cognitive and systems neuroscience approach, and a longitudinal study design. The specific aims of this clinical research proposal are to (1) Investigate longitudinal changes in cognitive and brain processes mediating mathematical cognition at grade 4, (2) Investigate longitudinal changes in functional and structural connectivity of brain networks underlying mathematical cognition, and (3) Investigate the cognitive and neural basis of individual differences in mathematical skill development. Our proposed studies will provide important insights into (1) the neural basis of functional specialization for mathematical cognition;(2) the relation between neural changes and changes in working memory, strategy and increased task proficiency;(3) the developmental origins of functional specialization in the posterior parietal cortex;(4) cortical networks and interactions that subserve mathematical task performance at different stages of skill development;and (5) cognitive and neural factors that mediate individual differences in learning. Taken together, this work will provide significant new information on the maturation of brain networks important for mathematical cognition and skill acquisition. This work has the potential to have broad impact on developmental cognitive neuroscience, brain development, and clinical psychology.